The present invention relates to a baseband signal communication apparatus which can modulate a baseband signal at a high S/N ratio with a simple circuit arrangement.
In a communication system disclosed, e.g., in Japanese Patent Laid-Open No. 56-140486, an interrogation signal wave and an energy wave composed of a carrier for a response signal wave are radio-transmitted from a fixed interrogator to a responsor carried by a user or attached to a moving object by using a microfrequency. Upon reception of these waves, the responsor converts the energy wave into an operation power and is operated. At the same time, the responsor modulates the carrier for a response signal wave with a response signal corresponding to the interrogation signal and radio-transmits the response signal wave to the interrogator with a weak electric field strength. In this communication system, interrogation and response signals are radio-transmitted as baseband signals having binary values of "1" and "0".
A conventional baseband signal communication apparatus used for the above-described communication system will be described with reference to FIGS. 6 to 9(h). FIG. 6 shows an interrogator of a communication system to which a reception means of the conventional baseband signal communication apparatus is applied. FIG. 7 shows a responsor of a communication system to which a transmission means of the conventional baseband signal communication apparatus is applied. FIGS. 8(a) to 8(e) show timing charts for explaining demodulation of a response signal wave from a first reception means in FIG. 1. FIG. 9(a) and FIGS. 9(f) to 9(h) show timing charts for explaining demodulation of a harmonic wave from a second reception means in FIG. 6.
An interrogator 1 in FIG. 6 will be described below. The interrogator 1 comprises a first oscillator 2 for generating a signal having a first frequency f1 (e.g., 2,440 MHz) in a microwave band, and a second oscillator 3 for generating a signal having a second frequency f2 (e.g., 2,455 MHz) which is slightly different from the first frequency f1. The signal having the first frequency f1 output from the first oscillator 2 is amplified by an amplifier 4. The amplified signal is then transmitted, as, e.g., a vertically-polarized energy wave, from an antenna 6 to a responsor (to be described later) through a circulator 5 without modulation. The signal having the second frequency f2 output from the second oscillator 3 is A1-modulated by a modulator 7 with an interrogation signal output from an arithmetic unit 8, and is amplified by an amplifier 9. The amplified signal is then transmitted, as an interrogation signal wave, from an antenna 10 to the responsor upon horizontal polarization.
Upon reception of the energy wave and the interrogation signal wave, the responsor (to be described later) transmits a response signal and a harmonic wave to the interrogator 1. The response signal is obtained as a first transmission signal wave by phase-modulating a carrier for the signal having the first frequency f1 with a response signal corresponding to the interrogation signal. The harmonic wave is obtained as a second transmission signal wave by amplitude-modulating a carrier for a second harmonic wave of the signal having the first frequency f1.
In the interrogator 1, the response signal wave is received by the antenna 6 and is supplied to a phase shifter 11 through the circulator 5. The response signal is branched by the phase shifter 11 into two signals whose carriers have a phase difference of 90.degree.. The two signals are respectively supplied to mixing circuits 12 and 13. Portions of the signal having the first frequency f1 from the first oscillator 2 are respectively supplied in phase, as detection carriers, to the mixing circuits 12 and 13. The mixing circuits 12 and 13 then output signals which are amplitude-modulated in accordance with a phase difference between the response signal wave and each detection carrier. The envelopes of the amplitude-modulated signals are detected by detectors 14 and 15, and baseband demodulated signals are respectively obtained as homodyne detection outputs. In addition, these demodulated outputs are amplified by amplifiers 16 and 17. The amplifiers 16 and 17 discriminate binary output levels of the demodulated signals as the baseband demodulated signals, and invert/output them if they have opposite phases. Level matching of the outputs from the amplifiers 16 and 17 is performed by level clamping circuits 18 and 19. The outputs are then supplied to an OR circuit 20. An OR output from the OR circuit 20 is supplied, as a first base band modulated signal, to a comparator 21. The mixing circuit 12 and the detector 14 constitute one homodyne detector as a demodulator, whereas the mixing circuit 13 and the detector 15 constitute the other homodyne detector as a demodulator. The antenna 6, the phase shifter 11, the two homodyne detectors, the amplifiers 16 and 17, the level clamping circuits 18 and 19, and the OR circuit 20 constitute the first reception means for receiving a phase-modulated response signal wave and outputting a first baseband demodulated signal.
The harmonic wave is received by an antenna 22 and is amplified by an amplifier 23. A second harmonic wave 2f1 (e.g., 4,880 MHz) of the signal having the first frequency f1 is extracted by a bandpass filter 24, and is supplied to a mixing circuit 25. The mixing circuit 25 receives a signal having a third frequency (e.g., 4,940 MHz) from a third oscillator 26. Frequency conversion is then performed by the mixing circuit 25 and an intermediate frequency signal (e.g., 60 MHz) is extracted by a bandpass filter 27. The intermediate frequency signal is amplified by an amplifier 28, and its envelope is detected by a detector 29, thus obtaining a baseband modulated signal. The detection output from the detector 29 is amplified by an amplifier 30, and is subjected to level matching in a level clamping circuit 31. The obtained signal is then supplied, as a second baseband demodulated signal, to the comparator 21. The antenna 22, the amplifiers 23, 28, and 30, the bandpass filters 24 and 27, the mixing circuit 25, the third oscillator 26, the detector 29, and the level clamping circuit 31 constitute the second reception means for receiving an amplitude-modulated harmonic wave and outputting a second baseband demodulated signal. The mixing circuit 25 and the detector 29 constitute a demodulator.
The comparator 21 compares the first baseband demodulated signal demodulated by the first reception means using the response signal wave with the second baseband demodulated signal demodulated by the second reception means using the harmonic wave. If they coincide with each other, the comparator 21 supplies either or the sum of the first and second baseband demodulated signals, as a demodulated signal of a baseband signal serving as a response signal, to the arithmetic unit 8. If they do not coincide with each other, the comparator 21 supplies an error signal to the arithmetic unit 8.
A responsor 40 in FIG. 7 will be described below. In the responsor 40, an interrogation signal wave from the interrogator 1 is received by an antenna 41, and is converted into an interrogation signal by a detector 42 and a low-pass filter 43. The signal is then amplified by an amplifier 44 and is supplied to an arithmetic unit 45. In addition, an energy wave from the interrogator 1 is received by an antenna 46 and is supplied to a phase modulator 47 and a rectifier 48. A carrier supplied to the phase modulator 47 is phase-modulated by a response signal output which is output from the arithmetic unit 45 in accordance with the interrogation signal, and a response signal wave as a first transmission signal wave is transmitted from an antenna 49 to the interrogator 1. The energy wave supplied to the rectifier 48 is converted into a DC operation power through a low-pass filter 50 and is supplied, as a driving power source, to the arithmetic unit 45 and the like. Upon rectification of the rectifier 48, harmonic components are generated, and a second harmonic wave is extracted by a bandpass filter 51. The second harmonic wave is then supplied, as a carrier, to an amplitude modulator 52. The second harmonic wave is amplitude-modulated by the amplitude modulator 52 in accordance with the response signal output from the arithmetic unit 45, and is transmitted, as a second transmission signal wave, from an antenna 53 to the interrogator 1.
In this arrangement, if a baseband signal as a response signal is represented by a waveform shown in FIG. 8(a), a carrier for an output from the phase modulator 47 is phase-shifted for one binary value but is not phase-shifted for the other value, as shown in FIG. 8(b). Upon reception of the response signal wave shown in FIG. 8(b), the first reception means of the interrogator 1 outputs first demodulated signals shown in FIGS. 8(c) and 8(d) by homodyne detection. If the baseband signal has a proper binary output level as shown in FIG. 8(c), it can be amplified as it is by the amplifier 16 as shown in FIG. 8(c'). If the output level has the opposite level as shown in FIG. 8(d), the baseband signal is inverted and amplified by the amplifier 17 as shown in FIG. 8(d'). In addition, the OR of the outputs from the amplifiers 16 and 17 is output, as a first baseband demodulated signal, from the OR circuit, as shown in FIG. 8(e).
In the output from the amplitude modulator 52, a second harmonic wave is output for one binary value and no output for the other binary value, as shown in FIG. 9(f). Upon reception of the harmonic wave shown in FIG. 9(f), the second reception means of the interrogator 1 outputs an intermediate frequency signal by frequency conversion, as shown in FIG. 9(g). In addition, an envelope detection output is output, as a second baseband modulated signal, from the detector 29, as shown in FIG. 9(h).
If the comparator 21 determines that the first and second baseband demodulated signals, which are transmitted from the first and second reception means with different carriers and by different modulation methods, coincide with each other, it is determined communication is properly performed. As a result, either or the sum of the first and second baseband demodulated signals is used, as a demodulated signal of the baseband signal serving as the response signal, in the arithmetic unit 8.
In the above-described communication system, a response signal wave transmitted from the responsor 40 to the interrogator 1 has a very weak electric field strength. For this reason, a response signal wave is susceptible to amplitude modulation due to a low-frequency electromagnetic wave having a frequency of 50 or 60 Hz generated by a fluorescent lamp located near the responsor 40. In addition, if the responsor 40 is moved, a carrier for a response signal received by the interrogator 1 is slightly frequency-modulated due to a Doppler effect. For this reason, a homodyne detection output as a first demodulated signal obtained by the first reception means of the interrogator 1 is equivalent to a baseband demodulated signal as a response signal on which low-frequency noise is superposed due to the above-mentioned amplitude modulation or frequency modulation. In order to decode the baseband demodulated signal, therefore, a certain circuit means for removing this low-frequency noise is required, resulting in a complicated circuit arrangement. In addition, decoding errors tend to occur.
Furthermore, the binary output level of a homodyne detection output of a response signal wave is inverted depending on the distance from the interrogator 1 to the responsor 40. Therefore, a circuit means for inverting a homodyne detection output depending on its binary output level is required. This complicates a circuit arrangement for decoding.
Moreover, in the first reception means, since the bandwidth of each of the amplifiers 16 and 17 is set to be wide in order to amplify a homodyne detection output having the same waveform as that of a baseband signal without distortion, it is difficult to realize a circuit arrangement which can increase the S/N ratio.
Similarly, in the second reception means, in association with a demodulating operation for obtaining a second baseband demodulated signal as a response signal from a harmonic wave, the bandwidth of the amplifier 30 is set to be wide in order to amplify an envelope detection output having the same waveform as that of a baseband signal without distortion. Therefore, it is difficult to realize a circuit arrangement which can increase the S/N ratio.
Especially, in a responsor having no operating power source, which is designed to externally receive a wave having a microfrequency or the like as an energy wave and to obtain an operation power source by converting the wave into a DC power, a response signal is transmitted with a weak electric field strength. Therefore, it is difficult to receive and demodulate a response signal at a high S/N ratio.